John Preskill on Quantum Computing

Key Takeaways

and what was the revelation that made made scientists and physicists think that a quantum computer could exist it's not obvious you know a lot of people thought he couldn't okay the idea that a quantum computer would be powerful was emphasized over 30 years ago by Richard Fineman the Caltech physicist and it was interesting how he came to that realization phiman was interested in computation his whole life ya know he had been involved during the war in Los Alamos he was the head of the computation group he was the guy who fixed the little mechanical calculators and he had a whole crew of people who were calculating and he figured out how to flow you know the work from one computer to another all that kind of stuff and as computing technology started to evolve you know he followed that and our in the 1970s mmmph particle physicists like Fineman that's my background too got really interested in using computers to study the properties of elementary particles like the quarks inside a nucleus you know we know proton isn't really a fundamental object it's got little beans rattling around inside but they're quantum and gell-mann who's good at names called them quarks yeah and now we have we've had a theory since the 1970s of how quarks behave and so in principle you know everything about the theory you can compute everything but you can't because it's just too hard and people started to simulate that physics with digital computers in the 70s and there are some things that they could successfully compute and some things they they couldn't because it was just too hard the resources required you know the memory the time we're out of reach and so Fineman in the early 80s said you know nature is quantum mechanical damage what a simulation of nature it should be quantum mechanical yeah you should use a quantum system to behave like another quantum system at the time called it a universal quantum simulator okay and now we call it a quantum computer and the idea caught on about ten years later when Peter shor made the suggestion that we could solve problems which don't seem to have anything to do with physics which are really things about numbers like finding the prime factors of a big integer and that caused a lot of excitement in part because the implications for cryptography are a bit disturbing but then physicists good physicists yeah started to consider can we really build this thing yeah and some concluded and argued fairly cogently that no you couldn't because of this difficulty that it's so hard to isolate systems from the environment well enough for them to behave quantumly and so it took a few years for that to sort out sort of at the theoretical level in the mid-90s we developed a theory called quantum error correction it's about how to encode the quantum state that you'd like to protect in such a clever way that even if there are some interactions with the environment that you can't control it still stays robust mm-hmm but at first that was just kind of a theorist fantasy it was a little too far ahead of the technology but you know 20 years later the technology is catching up so now this idea of quantum error correction has become something you can do in the lab yeah and how does quantum error correction work I've seen a bunch of diagram so maybe this is difficult to explain but how would you explain it well I would explain it this way I don't think I've seen it said the word entanglement yet yeah well I've been checking off all the bingo words yet okay so let's talk about entanglement because it's part of the answer to your question which I'm still not done answering what is quantum physics so what do we mean by entanglement it's really the characteristic way maybe the most important way that we know in which quantum is different from ordinary stuff a kind of classical what does it mean entanglement it means that you can have a physical system which has many parts which have interacted with one another so it's in kind of a complex correlated state of all those parts and when you look at the parts one at a time it doesn't tell you anything about the state of the whole thing the whole thing's in some definite state there's information stored in it you know you'd like to access that information let me be a little more concrete suppose it's a book okay okay it's a book it's a hundred pages long so if it's an ordinary book a hundred people could each take a page and read it they know what's on that page and then they could get together and talk and now they know everything that's in the book right but if it's a quantum book written in qubits where these pages are very highly entangled there's still a lot of information in the book but you can't read it the way I just described you can look at the pages one at a time but a single page when you look at it just gives you random gibberish it doesn't reveal anything about the content of the book why is that it's there's information in the book but it's not stored in the individual pages it's encoded almost entirely and how those pages are correlated with one another that's what we mean by quantum entanglement information stored in those correlations which you can't see when you look at the parts one at a time so you asked about quantum error correction yeah what's the basic idea it's to take advantage of that property of entanglement because let's say you have a system of many particles mm-hmm and the environment is kind of kicking them around it's interacting with them because you you can't really completely turn off those interactions no matter how hard you try but suppose we've encoded the information in entanglement so say if you look at one atom it's not telling you anything about the information you're trying to protect yeah so the environment isn't learning anything when it looks at the atoms one at a time and this is kind of the key thing that what makes quantum information so fragile is that when you look at it you disturb it this ordinary water bottle like that you know let's say we knew it was either here or here and we didn't know I would look at it I find out is here I was ignorant of where it was to start with now I know mm-hmm but with a quantum system when you look at it you you really change the state there's no way to avoid that so if the environment is looking at it in the sense that information is leaking out to the environment that's going to mess it up so we have to encode the information so the environment so to speak can't find out anything about what the information is and that's the idea of quantum error correction if we encoded an entanglement the environment is looking at the parts one at a time but it doesn't find out what the protected information is yeah no so in other words it's it's kind of measuring probability the whole way along right I'm not sure what you mean by that so is it Grover's algorithm that was bate like as a as quantum bits roll through go through gates the probability is determined of what what information is being passed through what's being computed yeah so Grover's algorithm is a way of sort of doing an exhaustive search through many possibilities okay I know like let's say I'm trying to solve some problem like you know a famous one is the Traveling Salesman problem I have told you what the distances are between all the pairs of cities and now I want to find the shortest route I can that visits them all that's a really hard problem and it's still hard for a quantum computer but not quite as hard because there's a way of solving it which is to try all the different routes and measure how long they are and then find the one that's shortest and you solved the problem the reason it's still hard to solve is there's such a vast number of possible routes what Grover's algorithm does is it speeds up that exhaustive search mm-hmm and in practice it's not that big a deal what it means is that you know if you have the same processing speed you can handle about you know twice as many say before the problem becomes too hard to solve as you could if you were using a classical processor mm-hmm but as far as what's quantum about grover it takes advantage of the property in quantum physics that probabilities might tell me if I'm getting too inside Bassman Oh perfect that probabilities are the squares of amplitudes this is interference and this another part of the answer we can spend the whole whole hour answering the question what does quantum physics another essential part of it is is what we call interference and and this is really crucial for understanding how quantum computing works and that is that probabilities add you know if you know the probability of one alternative and you know the probability of another then you can add those together and find the probability that one or the other occurred mm-hmm and it's not like that in quantum physics the famous example is the double slit interference experiment now I'm sending electrons let's say it could be basketballs but it's an easier experiment to do with electrons hmm at a screen and they're two holes in the screen and you can try to detect the electron on the other side of the screen mm-hmm and when you do that experiment many times you can plot a graph showing where the electron was detected you know in each run or make a histogram of all the different outcomes and the graph Wiggles okay so if it were if you could say there's some probability of going through the first hole and some probability of going through the second and each time you've detected it it went through either one or the other there'd be no Wiggles in that graph that's the interference that makes it wiggle and the essence of the interference is that nobody can tell whether it went through the first slit or the second slit the question is sort of inadmissible and this interference then occurs when we can add up these different alternatives in a way which is different from what we're used to it's not right to say that the electron was detected at this point because it had some probability of going through the first hole and some probability of going through the second we add those probabilities up that doesn't give the right answer the different alternatives can interfere mm-hmm and this is really important for quantum computing because what we're trying to do is enhance the probability or the time it takes to find the solution to a problem and this interference can work to our advantage we want to have you know when we're doing our search we want to have a higher chance of getting the right answer and a lower chance of getting the wrong answer and if the different wrong answers can interfere they can cancel one another out and that enhances the probability of getting the right answer so sorry it's such a long-winded no answer but this is how Grover's algorithm works so they can speed up exhaustive search by taking advantage of that interference phenomenon well this it's kind of one of the underlying questions among many of the questions from Twitter you've you've hit our record for most questions asked but basically many many people are wondering what quantum computers really will do if and when it becomes a reality that they outperform classical computers what are they going to be really good at well you know what I'm not really sure and I think you know if you look at the history of Technology it would be hubris to expect me to know it's a whole different way of dealing with information there's quantum information it's not just you know a quantum computer is not just a faster way of computing it deals with information in a completely new way because of this interference phenomenon because of entanglement that we've talked about and I think we have limited vision when it comes to predicting decades out what the impact will be of an entirely new way of doing things information processing in particular I mean you know this well if we go back to the 1960s and people are starting to put a few transistors on a chip where is that going to lead nobody knew even early days of the internet even the hood exam even the first browser hmm no one really knew what anyone was going to do with it you know it makes total sense for good or ill and but we have some ideas you know I think why are we confident there will be some transformative effect on Society of the things we know about and I emphasize again probably the most important ones are things we haven't thought of mmm when it comes to applications of quantum computing the ones which will affect everyday life I think are better methods for understanding and inventing new materials new chemical compounds mm-hmm things like that can be really important you know if you find a better way of capturing carbon by designing a better catalyst or you can design pharmaceuticals that have new effects materials that have unusual properties these are quantum physics problems because those property the molecule or the material really have to do with the underlying quantum behavior of the particles and we don't have a good way for solving such problems or predicting that behavior using ordinary digital computers that's what a quantum computer is good at it's good but maybe not the only thing it's good at but one thing it should certainly be good at is telling us quantitatively how quantum systems behave and in the two contexts I just mentioned there's little question that there will be practical impact of that so it's not it's not just doing the Traveling Salesman problem through the table of elements for like why it can find those compounds it's much more than if it were that would that wouldn't be very efficient exactly yeah no it's much trickier than that and you know the like I said the exhaustive search though conceptually yeah it's really interesting that quantum can speed it up because of interference from a practical point of view it may not be that big a deal it means that well like I said in the same amount of time you can solve an instance which is twice as big of the problem so what we rarely get excited about are the so-called exponential speed ups and that was why Shor's algorithm was excited exciting in in 1994 because factoring large numbers was a problem that had been studied by smart people for a long time and on that basis the fact that there weren't any fast ways of solving it was pretty good evidence it's a hard problem actually we don't know how to prove that from first principles maybe somebody will come away you know come along one day and figure out how to solve factoring very fast on a digital computer it doesn't seem very likely because people have been trying ok for so long to solve problems like that and it's just intractable with ordinary computers you could say the same thing about these quantum physics problems maybe some brilliant graduate student is going to drop a paper on the archive tomorrow which will say here I solve quantum chemistry and I can do it on a digital computer but we don't think that's very likely because we've been working pretty hard on these problems for decades and they seem to be really hard and so those cases like these number theoretic problems which have cryptological implications and tasks for simulating the behavior of quantum systems we're pretty sure those are hard problems classically and we're pretty sure quantum computers when we have algorithms that have been proposed but which we can't really run currently because our quantum computers aren't big enough on the scale that's needed to solve problems really people really care about yeah so so maybe we should jump to one of the questions from Twitter which is related to that so Travis Shelton asked what are the most problem pressings in physics let's say specifically around quantum computers that you think substantial progress ought to be made in to move the field forward I know Travis he was an undergrad oh okay are you doing Travis so the problems that we need to solve to you know quantum computing closer to realization at the level that would solve problems people care about well let's go over where we are now yeah definitely okay so people have been working on quantum hardware for you know 20 years working hard and there are a number of different approaches to building hardware and nobody really knows which is going to be the best we haven't we're I think we're far from collapsing to one approach which everybody agrees has the best long term prospects for scalability and so it's important that a lot of different types of hardware are being pursued and we can come back to what some of the different approaches are yeah later but so where are we now we we think in a couple of years we'll have devices with about 50 cubits 200 and we'll be able to control them pretty well and that's an interesting range because even though it's only 50 to 100 cubits doesn't sound like that big a deal but that's already too many to simulate with a digital computer even with the most powerful supercomputers today so from that point of view oh you know these are relatively small near-term quantum computers which we'll be fooling around with over the next five years or so are doing something that's kind of super classical yeah at least we don't know how to do exactly the same things with ordinary computers now that doesn't mean they'll be able to do anything that's practically important but we're gonna try okay we're gonna try and there are ideas about things we'll try out including sort of baby versions of these problems in chemistry and materials and ways of speeding up optimization problems nobody knows how well those things are going to work at these small scales part of the reason is not just that the number of qubits is small but they're also not perfect so we can perform elementary operations on pairs of qubits which we call quantum gates like the gates and ordinary logic but they have an error rate a little bit below you know an error every hundred gates so that if you have a circuit with a thousand cubits there's a lot of noise so exactly like it does for instance a hundred qubit quantum computer really mean a hundred quantum a hundred cubic quantum computer or do you need a certain amount of backup going on well I think in the near term we're going to be trying out and probably we have the best hopes the kind of hybrid classical methods with some kind of classical feedback okay you try to do something on the quantum computer you make a measurement that gives you some information then you change the way you did it a little bit and try to converge on some better answer and that's one possible way of addressing optimization that might be faster on a quantum computer but I just wanted to emphasize that the number of qubits isn't the only yeah metro yeah and how good they are and in particular the reliability of the gates how well we can perform them that's equally important so anyway coming back to Travis's question well there are lots of things yeah that we'd like to be able to do better but just having much better qubits would be huge right so if you more or less with the technology we have now you can have a gate error rate of a few parts and a thousand you know if you can improve that by orders of magnitude then obviously you could run bigger circuits and that would be very enabling even if you stick with a hundred qubits just by having a circuit with more depth you know more layers of gates that increases the range of what you could do mm-hmm so that's a and that's always going to be important yes I mean look at that's look at how crappy that is a gate error rate even if it's one part in a thousand it's pretty lousy compared to if you look at where and yes processor a billion transistors in it yeah and zero and you don't you don't worry about the it's gotten to the point where there is some error protection built in at a hardware level okay in a processor because I mean we're doing these crazy things like going down from 11 nanometer scale for features on a chip so so how are folks trying to deal with interference right now you mean what what types of devices and you know so that's interesting too because there are a range of different ways to do it so I mentioned that we could store information we can make a qubit out of a single atom for example that's one approach so you have to control a whole bunch of atoms and get them to interact with one another one way of doing that is with what we call trapped ions that means the atoms have electrical charges that's a good thing because then you can control them with electric fields you can hold them in a trap and you can isolate them like I said in a very high vacuum so they're not interacting too much with other things in the laboratory including stray electric and magnetic fields but that's not enough because you got to get them to talk to one another you got to get them to interact I mean we have this set of desiderata which are kind of in tension with one another on the one hand we want to isolate the qubits very well yeah on the other hand we want to control them from the outside and get them to do what we wanted to do and eventually we want to read them out you have to be able to read out the result of the computation but the key thing is the control if you want to do two of those qubits and your device to interact with one another in a specified way and do that very accurately you have to have some kind of bus that gets the - to talk to one another okay and the way they do that in an ion trap is pretty interesting it's by using lasers and controlling how the ions vibrate in the trap with a laser kind of excite Wiggles of the ion and then by you know determining whether the ions are wiggling or not you can go address another ion and that way you can do a two qubit interaction and you can do that pretty well okay another way is really completely different what I just described was encoding information at the one yeah atom level but another way is to use super conductivity circuits in which electric current flows without any dissipation and in that case you have a lot of freedom to sort of engineer the circuits to behave in a quantum way there's there are many nuances there but the key thing is that you can encode information now in a system that might involve the collective motion of billions of electrons and yet you can control it as though it were a single atom I mean here's one's oversimplified way of thinking about it yeah suppose you have a little loop of wire and there's current flowing in the loop it's a superconducting wire so it just keeps flowing normally there'd be resistance which would dissipate that as heat but not for the superconducting circuit which of course has to be kept very cold to stay superconducting but you can imagine in this little loop that the current is either circulating clockwise or counter clockwise so that's a way of encoding information but it could also be both at once and that's what makes it a qubit right and so in that case I'll even though it involves lots of particles the magic is that you can control that system extremely well I mentioned individual electrons that's another approach put the qubit and the spin yeah of a single electron you also mentioned better qubits what are you doing by that what would I what I really care about is how well I can do the gates yeah and there's a whole other approach which is motivated by the desire to have much much better control over the quantum information than we do in those systems that I mentioned so far like superconducting circuits and trapped ions that's actually what Microsoft is pushing very hard mm-hmm we call it topological quantum computing that's topological is so word physicists and mathematicians love it it means well let's we'll come back to what it means just tell you what they're trying to do they're trying to make a much much better Cuba which they can control much much better using a completely different hardware approach okay and it's very ambitious because at this point it's not even clear they have a single qubit but if that approach is successful and we're it's making progress so I think we will see a validated qubit of this type soon maybe next year okay and then I nobody really knows where it goes from there but suppose it's the case that you could do a two qubit gate with an error rate of 1 million instead of one and a thousand mm-hmm I mean that would be that would be huge now scaling all these technologies up really challenging from a number of perspectives including just the control engineering but so it was so via how are how are they doing it are attempting to do it you know you could ask um where did all this progress come from over 20 years or so for example with the superconducting circuits a sort of crucial measure is what we call the coherence time of the qubit which roughly speaking means how much it interacts with the outside world the longer the coherence time the better so the rate of what we call decoherence is essentially how much it's getting buffeted around by outside influences and for the superconducting circuits those coherence times have increased about a factor of 10 every 3 years going back 15 years or so Wow now it will necessarily go on like that indefinitely but in order to achieve that that type of progress better materials better fabrication better control the way you control these things is with microwave circuitry not that different from you know the kind of things that are going on and you know communication devices and all those things are important but I think going forward the control is is really the critical thing coherence times are already getting pretty long I mean having them longer it's certainly good but the key thing is to get to qubits to interact just the way you want them to and even if there is no I keep saying the key thing is the environments not the only key thing yeah because you know you have some qubit like if we if you think about that electron spin one way of saying it is I said it can be both up and down at the same time well there's a simpler way of saying that it might not point either up or down it might point some other way but the early a continuum ways of ways it could point that's not like a bit you see it's much easier to stabilize a bit because it's got just two states yeah but if it can kind of wander around in the space of possible configurations for a qubit that makes it much harder to control mm-hmm and you know people have gotten better at that a lot better at that in the last few years interesting so Joshua Herrmann asked what engineering strategy for quantum computers do you think has the most promise yeah so I mentioned some of these different approaches and I guess I'll interpret the question as which one is the winning horse yeah I know better than to answer that question they're all interesting okay but I for the near term the most advanced are superconducting circuits and trapped ions which is why I mentioned those first and I think that will remain true you know over the next five to ten years there are other technologies have the potential like these topologically protected humans to surpass those but it's not gonna happen real soon I kind of like superconducting circuits because there's so much um phase space of things you can do with them you know of ways you can engineer and configure them and imagine scaling them up they have the advantage of being faster than the time to take the cycle time the time to do a gate is faster than with the trapped ions just the basic physics of the interactions is different in the long term those electron spins could catapult ahead of these other things that's something that you can naturally do in silicon and you know it's potentially easy to enter gate integrate with silicon technology right now the you know qubits and gates aren't as good as the other technologies but that can change and I mean that from a theorist perspective this topological approach is very appealing and so we could imagine you know it takes off maybe ten years from now and it becomes the leader so I think it's important to emphasize we don't really know what's going to scale the best right and are there multiple attempts being made around programming quantum computers yeah I mean some of these companies yeah that are working on quantum technology now which includes well-known big players like IBM and Google and Microsoft and Intel but also a lot of startups now they are trying to encompass the full stack so they're interested in the hardware and the fabrication and the control technology but also the software the applications the user interface mm-hmm all those things are certainly going to be important eventually yeah they're pushing it almost to like an AWS layer where you you have you interact with your quantum computer in a server farm and you don't even touch it yeah it seemed I think that that's how it will be in the near term I think you you're not going to have most of us won't have a quantum computer you know sitting on your desktop or in your pocket maybe someday in the near term it'll be on the cloud and you'll be able to run applications on it by some kind of web interface and you know ideally that should be designed so the user doesn't have to know anything about going on physics in order to program or use it and I think that's part of what some of these companies are moving toward do you do you think it will get to the level where it's in your pocket how do you deal with that when you you're below 1 Kelvin well if it's in your pocket it probably won't be 1 Kelvin oh yeah probably none so what do you do well there's one approach as an example which I guess I mentioned in passing before or maybe it doesn't have to be at such low temperature and that's nuclear spins because they're very weakly interacting with the outside world you can have quantum information in a nuclear spin which I mean ideal you I'm not saying that it would be undisturbed for years but seconds which is pretty good okay um and you know you can imagine that getting significantly longer you know someday you might have a little quantum smart card in your pocket the nice thing about that particular technologies you can do it at room temperature still I'm long coherence times and you know if you're if you go to the ATM and you're worried that there's a rogue Bank it's gonna steal your information one solution to that problem I'm not saying there aren't other solutions is to have a quantum card where the bank will be able to authenticate it without being able to forge it we should talk about the security element Kevin su asked what risk would quantum computers pose to current encryption schemes so public key and what changes should people be thinking about if quantum computers come and that you know the next five years ten years yeah quantum computers threatened ecosystems that are in widespread use whenever you're using a web browser and you see that little padlock and you're in a you know HTTPS site you're using a public key cryptosystem to protect your privacy and those crypto systems rely for their security on the presumed hardness of computational problems that is it's possible to crack them but it's just too hard so RSA which is one of the ones that's widely used mmm-hmm as typically practiced today the to break it you'd have to do something like factor a number which is over 2,000 bits long to 2048 and that's you know that's too hard to do now mm-hmm but that's what quantum computers will be good at another one that's widely used is called a elliptic curve cryptography doesn't really matter exactly what it is okay but the point is that it's also a vulnerable to quantum attack yeah so we're gonna have to protect our our privacy in different ways when quantum computers are prevalent what what are the attempts being made right now well I mean there are two main classes of attempts okay one is just to come up with a cryptographic protocol not so different conceptually from what's done now but based on a problem that's hard for quantum yeah and it turns out that what has sort of become the standard way doesn't have that feature and there are alternatives that people are working on how we speak of post quantum crypt geography meaning the protocols that we'll have to use when we're worried that our adversaries have quantum computers mm-hmm and I don't think there's any proposed crypto system although there's a long list of them by now which people think are candidates for being quantum resistant for being unbreakable or hard to break by quantum computers I don't think there's anyone that you know the world has sufficient confidence in now that it's really hard for a quantum adversary that we're all going to switch over but it's certainly time to be thinking about it you know when people worry about the privacy of course different users have different standards but the US government sometimes says they would like a system to stay secure for 50 years they'd like to be able to use it for 20 roughly speaking and then have the intercepted traffic be protected for another 30 after that so I don't think though I could be wrong that were likely to have quantum computers that can break those public key cryptosystems in ten years mm-hmm but in 50 years seems not unlikely and so we should really be worrying about it and the other one is actually using quantum communication for privacy oh yeah so in other words if you and I could send qubits to one another instead of bits it opens up new possibilities so the way to think about these public key schemes are one way that we're using now is I want you to send me a private message and I can send you a lock box or foot know it has a padlock on it and but I keep the key okay but you can close up the box and send it to me mm-hmm but I'm the only one with the key so the key thing is that if you have the padlock you can't reverse engineer the key of course it's a digital box and key you know that's the idea of public key the idea of what we call quantum key distribution which is a particular type of quantum cryptography is that I can actually send you the key or you can send me your key but why can't any eavesdropper then listen in and know the key well it's because it's quantum and remember it has that property that if you look at it you disturb it so if you collect information about my key or if the adversary does that will cause some change in the key and there are ways in which we can check whether what you received is really what I sent mm-hmm and if it turns out it's not or it has too many errors in it then we'll be suspicious that there was an adversary who tampered with it and then we won't use that key because we haven't used it yet we're just trying to establish the key so we do the test to see whether an adversary interfere if it passes the test then we can use the key and if it fails the test we throw that key away and we try again that's how quantum cryptography works but it requires a much different infrastructure than what we're using now we have to be able to send qubits well it's not completely different because you can do it with photons and of course that's how we you know communicate through optical fiber now we're sending photons and it's a little trickier sending quantum information through an optical fiber because of that issue that interactions with the environment can disturb it but nowadays you know you can send quantum information through an optical fiber over the tens of kilometers with you know a low enough error rate so it's useful for communication Wow of course we'd like to be able to scale that up to global distances sure and their big challenge isn't that but anyway so that's that's another approach to the future of privacy that people are interested in and does that necessitate quantum computers on both ends - yes but not huge ones okay and the reason well yes and no okay at the scale of tens of kilometers no and that's you can do that now there are prototype systems that are in existence and but if you really want to scale it up then in other words to send things longer distance then you have to bring this quantum error correction idea into the game okay because at least with you know our current photonics technology there's no way I can send a single photon from here to China without there being a very high probability that gets lost in the fibers somewhere so we have to have what we call quantum repeaters okay which can boost the signal but it's not like the usual type of repeater that we have in communication networks now the usual type is you measure the sync signal and then you resend it okay I won't work for quantum because as soon as you measure it you're gonna mess it up so you have to find a way of boosting it without knowing what it is and of course it's important that it works that way because otherwise the adversary could just intercept it and resend it and so it will require some quantum processing to get that quantum error correction in the quantum repeater to work yeah but it's a much more modest scale quantum processor than we would need to solve hard problems okay gotcha and what are the other things that you're you're both excited about and worried about for for potential business opportunities Sneha I mispronounce names all the time Sneha and cat curry asks budding entrepreneurs what should they be thinking about in the context of quantum computing yeah I mean there's more to quantum technology than computing yeah and something which has good potential and to have an impact you know in the relatively near future is improved sensing hmm quantum systems it partly because of that property that I keep emphasizing that they can't be perfectly isolated from the outside they're good at sensing things and sometimes you know you want to detect it there's something in the outside world messes around with your qubit yeah and again using this technology of nuclear spins which I mentioned you can do it at room temperature potentially you can make a pretty good sensor and it can potentially achieve higher sensitivity and spatial resolution and look on look at things on shorter distance scales than than other existing sensing technology so one of the things people are excited about are the biological and medical implications of that if you can monitor the behavior of molecular machines you know probe biological systems at the molecular level using very powerful sensors and that would surely have a lot of applications so one interesting question you can ask is can you know use these quantum error correction ideas to make those sensors even more powerful and that's another area of you know current basic research but where you could see you know significant potential economic impact interesting and so and so in terms of your research right now what are you working on that you find both interesting and incredibly difficult everything I will find is 100% Oersted and incredibly difficult okay well let me um change direction a little yeah from what we've been talking about so far well let me tell you a little bit about me sure so I didn't start out interested in information yeah a career you know I'm a physicist and I was trained as no an elementary particle theorist studying the fundamental interactions and the elementary particles and that drew me into an interest in gravitation because one thing that we still have a very poor understanding of is how gravity fits together with the other fundamental interactions the way physicists usually say it is we don't have a quantum theory of gravity at least not one that we think is complete and satisfactory so I'm an interested in that question for you know many decades and then I kind of got sidetracked because I got excited about quantum computing but you know I I've always looked at quantum information not just as a technology you know I'm a physicist I'm not an engineer I'm not trying to build a better computer necessarily though I think that's very exciting and worth doing and if my work can contribute to that it's very pleasing but I see quantum information as a new frontier in the exploration of the physical sciences sometimes I call it the entanglement frontier you know we physics we like to talk about frontiers short distance frontier that's what we're doing at CERN you know and the Large Hadron Collider trying to discern new properties of matter at distances which was shorter than we've ever been able to explore before mm-hmm and there's a long distance frontier in cosmology you know we're trying to look deeper into the universe and understand its structure and behavior at earlier times those are both very exciting frontiers this entanglement frontier I think is increasingly going to be at the forefront of basic physics research in the 21st century mm-hmm and by entanglement frontier I just mean scaling up quantum systems to larger and larger complexity where it becomes harder and harder to simulate those systems you know with our existing digital tools and so that means we can very well anticipate the types of behavior that we're going to see I think that's a great opportunity for new discovery and that's part of what's going to be exciting even in the relatively near term mmm-hmm when we have a hundred qubits you know there are some things that we can do to understand the behavior of the dynamics of you know a highly complex system of 100 qubits that we know have been able to experimentally probe before and that's going to be very interesting but what we're starting to see now is that these quantum information ideas are connecting to these fundamental questions about gravitation and how to think about it quantum Li and it turns out as is true for most of the broader implications of quantum physics the key thing is entanglement and we can think of the microscopic structure of space-time the geometry of where we live geometry this means you know who's close to who else and okay if we are we're in the auditorium and you know I'm in the first row and you're in the fourth row you know the geometry is how close we are to one another so of course that's very fundamental in both space and time how far apart are we in space how far apart are we at a time is geometry really a fundamental thing or is it something that's kind of emergent from some even more fundamental concept it seems increasingly likely that it's really an emergent B property okay there's something deeper than geometry what is that we think it's quantum entanglement that you can think of the geometry as arising from quantum correlations among parts of a system and that's really what defines who's close to who and so we're trying to explore that idea more deeply and one of the things that comes in is the idea of quantum error correction mm-hmm remember the whole idea of quantum error correction was that we could make a quantum system behave the way we wanted to because it's well-protected against the damaging effects of noise and it seems like quantum error correction is part of the deep secret of how space-time geometry works it has a kind of intrinsic robustness coming from these ideas of quantum error correction that makes space you know meaningful so that it doesn't just evaporate when you when you tap on it if you wanted to you know you could think of the space time that the space the urine and the space that that I'm in as parts of a system that are entangled with one another mm-hmm is what would happen if we broke that entanglement and you know you're part of space became disentangled from my part Oh what we think that would mean is that there'd be no way to connect us anymore there wouldn't be any path through space that starts over here with you and ends with you would become broken apart into two pieces so it's really the entanglement which holds space together which keeps it from falling apart into little pieces and you know we're trying to get a deeper grasp of what that means and how do you make any progress on that that seems like the most unbelievably difficult problem to work on it's difficult yeah as well for a number of reasons but in particular because it's hard to get guidance from experiment which is how physics historically all science has commenced yeah and although it was fun a moment ago to talk about what would happen if we disentangled you're part of space from mine I don't know how to do that in the lab right now so of course part of the reason is we have the audacity to think we can figure these things out just by thinking about them maybe that's not true nobody knows right we should try yeah under solving these problems is a great challenge and you know it may be that the Apes that evolved on earth are not don't have the capacity to understand things like the quantum structure of space-time but maybe we do so we should try now in the longer term and maybe not such a long term I think maybe we can get some guidance from experiment and in particular what we're gonna be doing with quantum computers and you know the other quantum technologies that are becoming increasingly sophisticated in the next couple of decades as we'll be able to control very well highly entangled complex quantum systems and so that should mean that in a laboratory on a tabletop I can sort of make my own little toy spacetime hmm with an emergent geometry arising from the properties of that entanglement and I think that'll teach us lessons because systems like that are the type of system that because they're so highly entangled digital computers can't simulate them it seems like only quantum computers are potentially up to the task so that won't be quite the same as you know disentangling your side of the room for mine in real life yeah but we'd be able to do it in a laboratory setting you know using model systems which i think would help us to understand the basic principles better wildd yeah desktop space-time seems pretty cool yeah it's pretty fundamental we didn't really talk about what people sometimes we didn't implicitly but not in something we didn't talk about what people sometimes call quantum locality okay and it's another way of describing quantum entanglement actually there's this notion of Bell's Theorem that when you look at the correlations among the parts of a quantum system that they're different from any possible classical correlations and some things that you read give you the impression that you can use that to instantaneously send information over long distances hmm it is true that if we have two qubits electron spin say and they're entangled with one another then what's kind of remarkable is that I can measure my qubit to see along some axis whether it's up or down and you can measure yours and we will get perfectly correlated results no what I when I see up you'll see up say and when I see down you'll see down and sometimes people make it sound like that's remarkable that's not remarkable in itself I could have somebody could have flipped a pair of coins you know so they came up both heads and both tails and given one slip them afire to me yeah and go on a light you're opposed and then we do and then they call it quantum transport a teleportation on YouTube yeah of course what's really important about entanglement that makes it different from just those coins yeah is that there's more than one way of looking at a qubit mm-hmm you know we have what we call complementary ways of measuring it sorry you know you can ask whether it's up or down along this axis or along that axis there's nothing like that for the coin there's just one way to look at it hmm and what's cool about entanglement is it will get perfectly correlated results if we both measure in the same way mm-hmm but there's more than one possible way that we can measure and so what sometimes gets said or the impression Oh God is that that means that when I do something to my cubit it instantaneously affects your cubit even if we're on different sides of the galaxy but that's not what entanglement does it just means they're correlated in a certain way mm-hmm and when you look at yours if we have maximally entangled qubits you you just see a random Fett you know it could be a zero or one each occurring with probability 1/2 and that's going to be true no matter what I did to my Cupid and so you can't tell what I did by looking at it it's only that if we compare notes later we can see how they're correlated and that correlation holds for either one of these two complementary ways in which we could both measure and it's that fact that we have these complementary ways to measure that makes it impossible for a classical system to reproduce those same correlations mm-hmm so that's one misconception that's pretty widespread another one is this about quantum computing which is in trying to explain why quantum computers are powerful people will sometimes say well it's because you can superpose I use that word before you know you can add together many different possibilities and that means that whereas an ordinary computer would just do a computation once acting on a superposition and quantum computer can do a vast number of computations all at once there's a certain sense in which that's mathematically true if you interpret it right but it's very misleading because in the end you're gonna have to make some measurement to read out the result mm-hmm and when you read it out you're you know there's a limited amount of information you can get you're not going to be able to read out the results of some huge number of computations in a single shot measurement so really the key thing that makes it work is this idea of interference which we discussed briefly when you asked about Grover's algorithm mm-hmm the art of a quantum algorithm is to make sure that the